1. Field of the Invention
The present invention relates to an Internet Protocol (abbreviated to IP) packet transfer service network (abbreviated to IP network) having as a core network, an ATM network which transfers data in an Asynchronous Transfer Mode (ATM). It relates particularly to an apparatus and a method for selecting a router's communication protocol (simply called a protocol) for transferring an IP packet between the access routers.
Recently, communication systems have spread widely, which use the ATM network for network services and IP interchanges packets (e.g., data for use in computer communications) in a layer above the ATM layer of the Open Systems Interconnection (OSI). There are various methods for transferring an IP packet through the ATM network. Whichever method is used, a highly reliable method for transferring IP packets is in great demand.
2. Description of the Related Art
FIG. 1 is a diagram showing the configuration of an IP network system. Of the system components constituting the network system, only the components related to the present invention are shown in the figure.
There are two types of access routers: a type-A router provided at a user site and a type-B router offered by the network. Both accommodate terminal equipment, etc., (hereinafter called an IP user) of the IP packet transfer service network (IP network) and performs routing processing for directing the packet to/from the IP users.
The access router is assigned with a particular address (called ATM address) for identification in the network. Also, the IP user is assigned with a particular address (called IP address) for identification in the network.
An ARP/NHRP server (general term of an ARP server and an NHRP server) manages the access router's ATM address corresponding to the IP address of the IP user accommodated in the access router. The ARP server functions alone in the network, whereas the NHRP server which is a type of the ARP server, functions in a wide-area network that is divided into plural core networks, each including an ARP server and manages the ATM address of an access router existing in other core network by communicating with the ARP server in charge of the other core network.
A CLSF (Connectionless Service Function) server, which is provided in the ATM network, performs connectionless data exchange processing including routing processing, according to a connectionless data protocol. In the ATM network, the CLSF servers have fixed interconnection called ATM-PVC (Permanent Virtual Circuit) to transfer data in the ATM mode therebetween.
FIG. 2 shows the types and configuration of the access routers. There are three protocols used by the access routers: CLNAP (Connectionless Network Access Protocol, called method 1), IP-over-ATM (called method 2) and pure-ATM (called method 3). Thus, according to the protocol used, there are four types of access routers: CLNAP type, IP-over-ATM type, pure-ATM type and ALL type which uses all of the three protocols.
In the figure, an element enclosed in a box within the access router is a control program or a control circuit for executing each protocol. For example, the "ARP/NHRP" obtains from the aforesaid ARP/NHRP server, the ATM address of a destination access router corresponding to a destination IP address to which a packet is to be directed. The "ATM-SVC" establishes a connection called an ATM-SVC (Switched Virtual Circuit) based on the ATM address, with a destination access router in the ATM mode. The "P" transfers an IP packet in the ATM mode via the thus-established connection. FIG. 3 is a schematic diagram illustrating a conventional communication system. An example is shown here in which a forwarding access router having the CLNAP protocol (method 1) installed attempts to communicate with a destination access router having no CLNAP protocol installed.
On receiving from an originating IP user an IP packet to be transferred to a destination IP user having an IP address, the forwarding access router first obtains from the ARP/NHRP server (not shown), the ATM address of the destination access router accommodating the destination IP user having the IP address (hereinafter called the ATM address corresponding to the destination IP address). It then transmits the IP packet to the destination access router having the thus-obtained ATM address assigned, according to the aforesaid CLNAP protocol.
However, since the destination access router has not the CLNAP protocol installed, the transmitted IP packet cannot be received by the destination access router and is lost within the network. Moreover, since there is no way for the forwarding access router to recognize the lost IP packet, it continues to transmit IP packets in succession, eventually losing all the IP packets transmitted and finally disabling communication.
FIG. 4 is a schematic diagram illustrating another conventional communication system. An example is shown here in which a forwarding access router having the IP-over-ATM protocol (method 2) installed attempts to communicate with a destination access router having no IP-over-ATM protocol installed.
First, the forwarding access router establishes a connection (ATM-SVC) with the destination access router by using the ATM address which was obtained from the ARP/NHRP server (not shown). Then, it transmits the IP packet onto the thus-established connection in the ATM mode (i.e., according to the IP-over-ATM protocol), while encapsulating the IP packet into the AAL-PDU according to the RFC1483.
Here, it is assumed that the destination access router has the pure-ATM protocol installed instead of the IP-over-ATM protocol and has the IP protocol not defined in a higher-layer user protocol, as is the case with dynamic image data transferred on the ATM connection. Then, the destination access router can receive the transmitted AAL-PDU (Protocol Data Unit) payload but cannot interpret the payload because the aforesaid LLC is not defined, causing the same result as losing the packet within the network. Moreover, since there is no way for the forwarding access router to recognize the lost IP packet, it continues to transmit IP packets in succession, eventually losing the transmitted IP packets and finally disabling communication.
In the conventional method as described above, it is a problem that the transmitted packets are lost within the network and commutation is disabled in case the protocols disagree between the forwarding and destination access routers, in an IP network which is a wide-area public network accommodating a great many and unspecified IP users and which includes as a core network an ATM network having various protocols available for transferring IP packets.